(a) Field of the Invention
This invention relates to exhaust gas purifiers for diesel engines.
(b) Description of the Related Art
One of technical challenges for diesel combustion is to significantly reduce NOx and soot while maintaining good fuel economy. There is a known diesel combustion system capable of significantly reducing both of NOx and soot by recirculating a large amount of exhaust gas by EGR to reduce the combustion temperature and by injecting fuel relatively early for well mixture with air and causing the premixture to autoignite at the end of every compression stroke (see, for example, Published Japanese Patent Application No. 2001-82233).
On the other hand, an ideal combustion condition from the aspect of fuel economy is to accurately control the ignition timing to near the top dead center on each compression stroke in order to minimize the total sum of time loss, cooling loss and mechanical loss. According to the combustion system as described in the published document, however, a significant advance of the fuel ignition timing provides a very long ignition delay. Therefore, even if the fuel injection timing is changed to some extent, the ignition timing cannot be changed. Alternatively, if an attempt is made to control the ignition timing to near the top dead center on each compression stroke by increasing the amount of EGR to further extend the ignition delay time, this causes lack of oxygen to significantly increase HC and CO and thereby deteriorate the fuel economy. As a result, the combustion system as described in the published document cannot control the ignition timing to an appropriate timing.